The optical head device is required to emit short-wavelength light and to have an objective lens with a high numerical aperture (high NA) in order to support high-density optical disks.
Usually, an increase in the NA of the objective lens increases aberration caused by an error in a thickness of a transparent substrate covering an information recording surface of an optical disk or a thickness of a layer between recording layers in a multilayer recording disk. This degrades the quality of light beams converged on the recording surface of the optical disk, thereby degrading the recording/reproducing quality and exerting a great influence on the quality of a defocus detection signal of an objective lens against an optical disk and the quality of a tracking error detection signal for detecting a deviation of a light beam converged on the recording surface of the optical disk from a recording track formed on the optical disk. In order to maintain the quality of these detection signals, the aberration must be detected and corrected or an optical system insusceptible to aberration must be provided.
Especially, in an optical disk apparatus with a high-NA objective lens, a substrate thickness error of an optical disk or a layer thickness error of an optical disk having a multilayer structure causes a great spherical aberration to be generated, thereby worsening the quality of the light converging spot of the light beam. Moreover, if an optical disk apparatus with a high-NA objective lens uses a representative defocus detection method for an objective lens, which is an astigmatic method or a knife edge method, the spherical aberration considerably degrades amplitude of a defocus detection signal, thereby destabilizing the focus servo operation performed by using the defocus detection signal and also destabilizing the seek operation because a modulation signal by a light converging spot traversing a recording track is unavailable.
Spherical aberration in an optical system needs to be optically suppressed or corrected in order to avoid these problems. There is a proposal for suppressing or correcting spherical aberration of the light beam converged on an optical disk by disposing a liquid-crystal phase corrector element which can generate a spherical aberration in an optical path (refer to Patent Document 1, for example). Further, there is a proposed method for suppressing or correcting the spherical aberration by disposing a group of lenses that can generate a spherical aberration depending on a lens-to-lens distance in an optical path to vary a lens-to-lens distance in accordance with the amount of spherical aberration (refer to Non-Patent Document 1, for example).
It is important for an accurate correction of the spherical aberration to detect the amount of spherical aberration in the optical system. There is a proposed method for detecting the spherical aberration, in which an inside ray and an outside ray of the light beam reflected by the optical disk are separately detected by the astigmatic method and the amount of spherical aberration is detected from the difference between the focus error signals respectively obtained from the inside ray and the outside ray (refer to Patent Document 2, for example). Further, by adopting the knife edge method, instead of the astigmatic method, to the inside ray and the outside ray of the light beam reflected from the optical disk, the amount of spherical aberration can also be detected from the difference between the defocus signals respectively generated from the inside ray and the outside ray (refer to Patent Document 2, for example).
Patent Document 1: Japanese Patent Application Publication No. H10-269611 (paragraphs 0021-0022, FIG. 1, and FIG. 2)
Patent Document 2: Japanese Patent Application Publication No. 2002-367197 (paragraphs 0010, 0020 to 0022, FIG. 2, and FIG. 3)
Non-Patent Document 1: Tohru Kimura et al., “Optical system for HD-DVD with plastic lenses”, Optics-photonics Design and Fabrication (ODF) 2002, Tokyo, 1 Nov. 2002, Technical Digest, pp. 83-84
Non-Patent Document 2: Charles S. Williams and Orville A. Becklund, “Introduction to the optical transfer function”, WILEY-INTERSCIENCE, pp. 337-341